Abstract : The flights of A. Nikolaev and P. Popovich were conducted in order to determine the effects of unusual physical conditions (weightlessness, cosmic radiation, etc.) encountered in space on man's physiological functions, work capacity and mental status in prolonged space flights. It was also necessary to determine the effectiveness of preflight training, and to evaluate the accuracy of calculations made of man's requirements of food, water, and oxygen, and the suitability of the hygienic conditions in the spaceships cabin.